Created with NetLogo version NetLogo 3.1.4
Running with NetLogoLite.jar version 314.


This is a version of the Model Library model modifed to color the borders, rather than the fields, of the voroni cells.
The tricky bit there was that for a patch to know if it is on the border, the value of my-point in all the neighbor patches has to be updated first. So, in this version, there are two updater per direction, playing leapfrog, and waiting for the updaters to either side to update their patches.
It works well, and in "show-updates?" mode, is rather elegant looking.


This model draws a Voronoi diagram of polygons around a set of points. These diagrams resemble many phenomena in the world including cells, forest canopies, territories of animals, fur and shell patterns, crystal growth and grain growth, cracks in dried mud and other geological phenomena, road networks, and so on. Voronoi diagrams are useful in computer graphics, vision and path planning for robots, marketing, and other applications.


First the points are placed randomly. Then the polygons are drawn according to the following rules. Each point is enclosed inside exactly one polygon. All of the points inside the polygon are closer to that point than they are to any of the other points.
Instead of calculating the mathematically exact coordinates of the polygons, this model constructs an approximation using a grid. Each grid cell (each "patch", in NetLogo terminology) is colored according to which point it is closest to.


Use the NUMBER slider to choose how many points you want, then press SETUP. The model will place the points and draw the polygons.
If you want to play with moving the points around yourself, press the GO button. Now you can drag the points around with the mouse. As you move a point, the model redraws the polygon. This takes time, so it redraws them starting near the mouse and proceeding outward. If you want to see the boundary of the updated region, turn on the SHOW-UPDATES? switch.


The line segment separating two points is exactly midway between them.
How many sides do the polygons typically have? (You may want to ignore the polygons around the edges.)
Where different colors touch, there is usually a "Y". When do you get a "T" or an "X" instead?


Experiment with the effect of moving the points around. Moving the points slowly is best. (If you move them too fast, the model will have trouble keeping up and it won't be easy to see what's going on.)
Align two points so they have the exact same x coordinate or y coordinate. Is the line between them always perfectly smooth? (To see the effect, you may have to move the points closer or farther away from each other. Look closely.) Also try putting two points exactly on top of each other. What happens? Both effects occur because when a grid square ("patch") is equally distant from two differently colored points, NetLogo resolves the tie randomly.


Instead of placing the points completely randomly, have them move away from each other until they are roughly equidistant from each other. This makes all the polygons roughly the same size.
Edit the view and turn wrapping on in both directions, and click SETUP. The model may seem to be working, but there is a problem. If you turn on SHOW-UPDATES?, you can see that the update rectangle keeps going forever, continually refreshing the grid colors. Fix the model to work with wrapping, so that update stops as soon as the whole screen has been redrawn.
Instead of using the patches to display Voronoi polygons, find the boundaries by using turtles. Create a large batch of turtles at each point (colored the same color as the point), each turtle facing a different angle. Have the turtles walk outward from their points at a uniform rate. Stop the turtles when they run into a turtle of a different color.
Instead of using a patch-based approximation, calculate the exact positions of the sides of the polygons. (There are numerous published algorithms for calculating this information.) Then display the polygons using turtles with the "line" shape.


The core procedure for drawing the polygons is called "recolor"; it is only one line long! It puts the MIN-ONE-OF and DISTANCE reporters to good use.
The MOUSE-DOWN?, MOUSE-XCOR, and MOUSE-YCOR primitives are used so the user can interact with the model.
Because the number of patches is so large, it takes a while to update them all when a point moves. So we use moving turtles to recolor the patches; the moving turtles start where the mouse is, and move outwards in a square, since near the mouse is where the user will be looking first. See the Procedures tab for the details on how it works.


MaterialSim Grain Growth


For more information on Voronoi diagrams, see http://en.wikipedia.org/wiki/Voronoi. (There are also many other sites on this topic on the web.)
Thanks to John Jungck from Beloit College for inspiring this model with his talk at Northwestern University about Voronoi structures in nature.
Thanks to Josh Unterman and Seth Tisue for their work on this model.
To refer to this model in academic publications, please use: Wilensky, U. (2006). NetLogo Voronoi model. http://ccl.northwestern.edu/netlogo/models/Voronoi. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
In other publications, please use: Copyright 2006 Uri Wilensky. All rights reserved. See http://ccl.northwestern.edu/netlogo/models/Voronoi for terms of use.


NetLogo Version: NetLogo 3.1.4

;; Summary
;; Modified Model Library Model to show borders
;; See Copyright Notice in the code.
globals [
  available-colors      ;; list of colors that will be assigned to points
  current-point         ;; the point the user is currently moving

breed [points point]    ;; these are the little circles in the middle of the polygons

;; The next two breeds are used only when we're updating the polygons
;; when the user moves the points with the mouse.  See below for further
;; details.
breed [spawners spawner]
breed [updaters updater]

;; additions for border coloring
patches-own [ my-point updated ]
spawners-own [ update-time ]
updaters-own [ update-time spread? waiting? ]

;;; These are the only procedures necessary to draw the diagram
;;; initially, without moving points.

to setup
  ;; too dark and too light are hard to distinguish from each other,
  ;; so only use 13-17, 23-27, ..., 133-137
  set available-colors shuffle filter [(? mod 10 >= 3) and (? mod 10 <= 7)]
                                      n-values 140 [?]
  set-default-shape points "circle 3"
  ask n-of number patches [ make-point ]
  ;; additions for border coloring
  ask patches [ reassign set updated timer ]
  ask patches [ recolor ]
  set current-point nobody

to make-point ; patch procedure
  sprout-points 1 [
    set size 5
    set color first available-colors
    set available-colors butfirst available-colors

;; additions / changes for border coloring
to reassign  ;; patch or turtle procedure
  set my-point min-one-of points [distance myself]

to recolor  ;; can be patch or turtle procedure
  ifelse any? neighbors4 with [ my-point != my-point-of myself ] 
  [ set pcolor color-of my-point ]
  [ set pcolor black ]

;;; The rest of the procedures are used for efficiently updating
;;; the diagram when the user moves the points around.

to go
  ;; using EVERY here gives the updaters more time to do their thing
  ;; if the mouse is moving; without EVERY, the updates are too frequent
  ;; as the mouse moves and can be distracting
  every 0.2 [ obey-mouse ]
  [ ask spawners [ spawn ]
    ask updaters [ update ]

to obey-mouse
  ;; first handle the case where the user has released the mouse button
  ;; (or hasn't pressed it yet)
  if not mouse-down? [
    set current-point nobody
  ;; if the mouse button is down, get the mouse position
  let x round mouse-xcor
  let y round mouse-ycor
  ;; if we don't have a point yet, pick the closest one
  if current-point = nobody [
    set current-point min-one-of points [distancexy x y]
  ;; check if the point needs to move
  if x != xcor-of current-point or y != ycor-of current-point [
    ;; move the point
    ask current-point [ setxy x y ]
    ;; the point has moved, so we need to recolor all patches, so we kill off
    ;; the old turtles that were doing the recoloring and make new ones
    ask spawners [ die ]
    ask updaters [ die ]
    ask current-point [ ask patch-here [ make-spawners ] ]

;; Here's how we use turtles to update the patches in a growing
;; square pattern.  We use two breeds of turtles, spawners and updaters.
;; Spawners are at the corners of the square and move diagonally.  Each
;; time a spawner moves, it spawns two new updaters.  The updaters move
;; vertically or horizontally, and every time an updater lands on a patch,
;; it recolors it.  When a spawner or an updater hits the edge of the world,
;; it dies.  Together, these rules are enough to make the growing square!

to make-spawners  ;; patch procedure
  let counter 0
  set updated timer
  ; ask updaters [ set update-time [ updated ] of myself ]
  sprout-spawners 4 [
    ;; additions for border coloring
    set update-time updated
    ;; give the four headings of NE, SE, SW, and NE
    set heading 45 + counter * 90
    set counter counter + 1
    set color gray
    set shape "wide arrow"
    if not show-updates? [ hide-turtle ]

to spawn  ;; spawner procedure
   ;; additions for border coloring
  hatch-updaters 1 [ rt 45 if any? updaters-here with [ self != myself and heading = heading-of myself] [ die ] ]
  hatch-updaters 1 [ lt 45 if any? updaters-here with [ self != myself and heading = heading-of myself] [ die ] ]
  if not can-move? 1 [ die ]
  ;; Moving diagonally is a little tricky.  Moving forward 1 isn't enough by
  ;; itself, since that doesn't take us all the way to the center of the
  ;; diagonally next patch.  So after moving forward, we use SETXY to move
  ;; to the exact center of the new patch.
  fd 1
  setxy pxcor pycor

to update  ;; updater procedure
;; additions for border coloring
   ;; since only coloring border patches
   ;; we need to know if this is a border patch
   ;; before coloring, in order color this patch properly.
   ;; since being a border patch depends on the values of
   ;; the neighboring patches, we cant color this patch
   ;; until we know that neighbor patches have been updated
   ;; so this updater first checks if this patch has been reassigned yet,
   ;; if not, it reassigns the patch, and updates the update time in the patch
   ;; then, if it has not already, this updater hatches the updater
   ;; that will update the next patch in line
   ;; (it does this only once), then these two patches will play leap frog.
   ;; finally, this updater sits on this patch,
   ;; waiting for all the neighbor patches to get updated
   ;; (ie, it does nothing until all the neighbor patches has been updated)
   ;; once they get updated, this patch is recolored
   ;; and the updater dies.
  if updated < update-time
  [ reassign 
    set updated update-time 
  if spread? != true
  [ set spread? true
    if can-move? 1 
    [ hatch 1
      [ jump 1
        set color white
  ifelse any? neighbors4 with [ updated < updated-of myself ]
  [ recolor 
    ifelse can-move? 2
    [ jump 2 ]
    [ die ]

;; back converted to netlogo 3.1.4

; *** NetLogo 4.0beta1 Model Copyright Notice ***
; Copyright 2006 by Uri Wilensky.  All rights reserved.
; Permission to use, modify or redistribute this model is hereby granted,
; provided that both of the following requirements are followed:
; a) this copyright notice is included.
; b) this model will not be redistributed for profit without permission
;    from Uri Wilensky.
; Contact Uri Wilensky for appropriate licenses for redistribution for
; profit.
; To refer to this model in academic publications, please use:
; Wilensky, U. (2006).  NetLogo Voronoi model.
; http://ccl.northwestern.edu/netlogo/models/Voronoi.
; Center for Connected Learning and Computer-Based Modeling,
; Northwestern University, Evanston, IL.
; In other publications, please use:
; Copyright 2006 Uri Wilensky.  All rights reserved.
; See http://ccl.northwestern.edu/netlogo/models/Voronoi
; for terms of use.
; *** End of NetLogo 4.0beta1 Model Copyright Notice ***


Download Link

View or download the complete model file (to download: right-click, save-link-as):
-- Download voronoi-borders-model-libray-mod-netlogo3.1.4 --